Improving the Crab—more sophisticated programming€¦ · method is defined in another class, we need to specify the class or object that has the method, followed by a period (dot),

In the previous chapter, we looked at the basics of starting to program our first game. There weremany new things that we had to look at. Now, we will add more interesting behavior. Addingcode will get a little easier from now on, since we have seen many of the fundamental concepts.

The first thing we shall look at is adding some random behavior.

3.1 Adding random behaviorIn our current implementation, the crab can walk across the screen, and it can turn at the edgeof our world. But when it walks, it always walks exactly straight. That is what we want tochange now. Crabs don’t always go in an exact straight line, so let us add a little randombehavior: The crab should go roughly straight, but every now and then it should turn a littleoff course.

We can achieve this in Greenfoot by using random numbers. The Greenfoot environment itselfhas a method to give us a random number. This method, called getRandomNumber, expects aparameter that specifies the limit of the number. It will then return a random number between0 (zero) and the limit. For example,

Greenfoot.getRandomNumber(20)

will give us a random number between 0 and 20. The limit—20—is excluded, so the number isactually in the range 0–19.

The notation used here is called dot notation. When we called methods that were defined in ourown class or inherited, it was enough to write the method name and parameter list. When themethod is defined in another class, we need to specify the class or object that has the method,followed by a period (dot), followed by the method name and parameter. Since thegetRandomNumber method is not in the Crab or Animal class, but in a class calledGreenfoot, we have to write “Greenfoot.” in front of the method call.

topics: random behavior, keyboard control, sound

concepts: dot notation, random numbers, defining methods, comments

Concept:

When a methodwe wish to call isnot in our ownclass or inherited,we need to specifythe class or objectthat has themethod before themethod name,followed by a dot.This is called dotnotation.

CHAPTER

Improving the Crab—moresophisticated programming3

28 | Chapter 3 ■ Improving the Crab—more sophisticated programming

Let us say we want to program our crab so that there is a 10 percent chance at every step that thecrab turns a little bit off course. We can do the main part of this with an if-statement:

if ( something-is-true ){

turn(5);}

Now we have to find an expression to put in place of something-is-true that returns true inexactly 10 percent of the cases.

We can do this using a random number (using the Greenfoot.getRandomNumber method)and a less-than operator. The less-than operator compares two numbers and returns true if thefirst is less than the second. “Less than” is written using the symbol “<”. For example:

2 < 33

is true, while

162 < 42

is false.

Exercise 3.1 Before reading on, try to write down, on paper, an expression using thegetRandomNumber method and the less-than operator that, when executed, is true exactly10 percent of the time.

Exercise 3.2 Write down another expression that is true 7 percent of the time.

Concept:

Methods thatbelong to classes(as opposed toobjects) aremarked with thekeyword static intheir signature.They are alsocalled classmethods.

Note: Static methods

Methods may belong to objects or classes. When methods belong to a class, we write

class-name.method-name (parameters);

to call the method. When a method belongs to an object, we write

object.method-name (parameters);

to call it.

Both kinds of methods are defined in a class. The method signature tells us whether a givenmethod belongs to objects of that class, or to the class itself.

Methods that belong to the class itself are marked with the keyword static at the beginningof the method signature. For example, the signature of Greenfoot’s getRandomNumbermethod is

static int getRandomNumber(int limit);

This tells us that we must write the name of the class itself (Greenfoot) before the dot in themethod call.

We will encounter calls to methods that belong to other objects in a later chapter.

3.1 Adding random behavior | 29

Note

Java has a number of operators to compare two values. They are:

< less than >= greater than or equal> greater than == equal<= less than or equal != not equal

If we want to express the chance in percent, it is easiest to deal with random numbers out of 100.An expression that is true 10 percent of the time, for example, could be

Greenfoot.getRandomNumber(100) < 10

Since the call to Greenfoot.getRandomNumber(100) gives us a new random numberbetween 0 and 99 every time we call it, and since these numbers are evenly distributed, they willbe below 10 in 10 percent of all cases.

We can now use this to make our crab turn a little in 10 percent of its steps (Code 3.1).

Code 3.1Random coursechanges—first try

import greenfoot.*; // (World, Actor, GreenfootImage, and Greenfoot)

/*** This class defines a crab. Crabs live on the beach.*/public class Crab extends Animal{

public void act(){

if ( atWorldEdge() ){

turn(17);}

if ( Greenfoot.getRandomNumber(100) < 10 ){

turn(5);}

move();}

}

Exercise 3.3 Try out the random course changes shown above in your own version.Experiment with different probabilities for turning.

This is a pretty good start, but it is not quite nice yet. First of all, if the crab turns, it always turns thesame amount (5 degrees), and secondly, it always turns right, never left. What we would really like


Exercise 3.4 Try out the code shown above. What do you observe? Does the crab turndifferent amounts when it turns?

Exercise 3.5 We still have the problem that the crab turns only right. That’s not normalbehavior for a crab, so let’s fix this. Modify your code so that the crab turns either right or leftby up to 45 degrees each time it turns.

Exercise 3.6 Try running your scenario with multiple crabs in the world. Do they all turnat the same time, or independently? Why?

The project little-crab-2 (included in the book scenarios) shows an implementation of what wehave done so far, including the last exercises.

3.2 Adding wormsLet us make our world a little more interesting by adding another kind of animal.

Crabs like to eat worms. (Well, that is not true for all kinds of crab in the real world, but there aresome that do. Let’s just say our crab is one of those that like to eat worms.) So let us now add aclass for worms.

We can add new actor classes to a Greenfoot scenario by selecting New subclass from one of theexisting actor classes (Figure 3.1). In this case, our new class Worm is a specific kind of animal,so it should be a subclass of class Animal. (Remember, being a subclass is an is-a relationship:A worm is an animal.)

When we are creating a new subclass, we are prompted to enter a name for the class and to selectan image (Figure 3.2).

In our case, we name the class “Worm”. By convention, class names in Java should always startwith a capital letter. They should also describe what kind of object they represent, so “Worm” isthe obvious name for our purpose.

Then, we should assign an image to the class. There are some images associated with the scenario,and a whole library of generic images to choose from. In this case, we have prepared a worm imageand made it available in the scenario images, so we can just select the image named worm.png.

to see is that the crab turns a small, but random amount to either its left or its right. (We will discussthis now. If you feel confident enough, try to implement this on your own first before reading on.)

The simple trick to the first problem—always turning the same amount, in our case 5 degrees—is to replace the fixed number 5 in our code with another random number, like this:


turn( Greenfoot.getRandomNumber(45) );}

In this example, the crab still turns in 10 percent of its steps. And when it turns, it will turn a randomamount, between 0 and 44 degrees.

3.2 Adding worms | 31

Figure 3.1Creating newsubclasses

Figure 3.2Creating a new class


Exercise 3.7 Add some worms to your world. Also add some crabs. Run thescenario. What do you observe? What do the worms do? What happens when a crabmeets a worm?

We now know how to add new classes to our scenario. The next task is to make these classesinteract: When a crab finds a worm, it should eat it.

3.3 Eating wormsWe now want to add new behavior to the crab: When the crab runs into a worm, it eats it. Again,we first check what methods we have already inherited from the Animal class. When we openthe editor for class Animal again, and switch to the Documentation view, we can see thefollowing two methods:

boolean canSee (java.lang.Class clss)Return true if we can see an object of class ‘clss’ right where we are.

void eat (java.lang.Class clss)Try to eat an object of class ‘clss’.

Using these methods, we can implement this behavior. The first method checks whetherthe crab can see a worm. (It can see it only when it runs right into it—our animals are veryshort-sighted.) This method returns a boolean—true or false, so we can use it in an if-statement.

The second method eats a worm. Both methods expect a parameter of type java.lang.Class.This means that we are expected to specify one of our classes from our scenario. Here is somesample code:

if ( canSee(Worm.class) ){

eat(Worm.class);}

In this case, we specify Worm.class as the parameter to both method calls (the canSeemethod and the eat method). This declares which kind of object we are looking for,and which kind of object we want to eat. Our complete act method at this stage is shown inCode 3.2.

Try this out. Place a number of worms into the world (remember: shift-clicking into the world isa shortcut for quickly placing several actors), place a few crabs, run the scenario, and see whathappens.

Once done, we can click Ok. The class is now added to our scenario, and we can compile andthen add worms to our world.

3.4 Creating new methods | 33

Code 3.2First version of eatinga worm

public void act(){


turn(17);}


turn(Greenfoot.getRandomNumber(90)-45);}move();


eat(Worm.class);}

}

Advanced note: Packages

(The notes labeled “Advanced note” are inserted for deeper information for those readers reallyinterested in the details. They are not crucial to understand at this stage, and could safely beskipped.)

In the definition of the canSee and eat methods, we have seen a parameter type with thename java.lang.Class. What is going on here?

Many types are defined by classes. Many of those classes are in the standard Java classlibrary. You can see the documentation of the Java class library by choosing Java LibraryDocumentation from Greenfoot’s Help menu.

The Java class library contains thousands of classes. To make these a little easier to work with,they have been grouped into packages (logically related groups of classes). When a classname includes dots, such as java.lang.Class, only the last part is the name of the classitself, and the former parts form the name of the package. So here we are looking at the classnamed “Class” from the package “java.lang”.

Try to find that class in the Java library documentation.

3.4 Creating new methodsIn the previous few sections, we have added new behavior to the crab—turning at the edge of theworld, occasional random turns, and eating worms. If we continue to do this in the way we havedone so far, the act method will become longer and longer, and eventually really hard to under-stand. We can improve this by chopping it up into smaller pieces.

We can create our own additional methods in the Crab class for our own purposes. For exam-ple, instead of just writing some code that looks for a worm and eats it into the act method, wecan add a new method for this purpose. To do this, we first have to decide on a name for thismethod. Let us say we call it lookForWorm. We can then create a new method by adding thefollowing code:

/*** Check whether we have stumbled upon a worm.* If we have, eat it. If not, do nothing.*/

public void lookForWorm(){


eat(Worm.class);}

}

The first four lines are a comment. A comment is ignored by the computer—it is written forhuman readers. We use a comment to explain to other human readers what the purpose of thismethod is.

When we define this method, the code does not immediately get executed. In fact, by just definingthis method, it does not get executed at all. We are just defining a new possible action (“looking fora worm”) that can be carried out later. It will only be carried out when this method is called. Wecan add a call to this method inside the act method:

lookForWorm();

Note that the call has the parentheses for the (empty) parameter list. The complete source codeafter this restructuring is shown in Code 3.3.


Concept:

Comments arewritten into thesource code asexplanations forhuman readers.They are ignoredby the computer.

Code 3.3Splitting code intoseparate methods

public void act(){


turn(17);}


turn(5);}

move();lookForWorm();

}

Concept:

A method defini-tion defines a newaction for objectsof this class. Theaction is not imme-diately executed,but the methodcan be called witha method call laterto execute it.

3.4 Creating new methods | 35

/*** Check whether we have stumbled upon a worm.* If we have, eat it. If not, do nothing.*/public void lookForWorm(){


eat(Worm.class);}

}

Code 3.3 continuedSplitting code intoseparate methods

Note that this code change does not change the behavior of our crab at all. It just makes the codeeasier to read in the long run. As we add more code to the class, methods tend to become longerand longer. Longer methods are harder to understand. By separating our code into a number ofshorter methods, we make the code easier to read.

Exercise 3.8 Create another new method named randomTurn (this method has noparameters and returns nothing). Select the code that does the random turning, and moveit from the act method to the randomTurn method. Then call this new randomTurnmethod from your act method. Make sure to write a comment for this method.

Exercise 3.9 Create yet another method named turnAtEdge (it also has no parametersand returns nothing). Move the code that checks whether we are at the edge of the world (anddoes the turn if we are) into the turnAtEdgemethod. Call the turnAtEdgemethod from youract method. Your act method should now look like the version shown in Code 3.4.

Code 3.4The new act methodafter creatingmethods for the subtasks

public void act(){

turnAtEdge();randomTurn();move();lookForWorm();

}

By convention, method names in Java always start with a lowercase letter. Method namescannot contain spaces (or many other punctuation characters). If the method name logicallyconsists of multiple words, we use capitals in the middle of the method name to mark the startof each word.


Exercise 3.10 Add a new class to your scenario. The class should be a subclass ofAnimal, called Lobster (with a capital ‘L’), and it should use the prepared image lobster.png.

Exercise 3.11 What do you expect lobsters to do when you place them into the world asthey are? Compile your scenario and try it out.

We now want to program our new lobsters to eat crabs. This is quite easy to do, since the behav-ior is very similar to the behavior of crabs. The only difference is that lobsters look for crabs,while crabs look for worms.

Exercise 3.12 Copy the complete act method from the Crab class into the Lobsterclass. Also copy the complete lookForWorm, turnAtEdge, and randomTurn methods.

Exercise 3.13 Change the Lobster code so that it looks for crabs, rather than worms.You can do that by changing every occurrence of “Worm” in the source code to “Crab”. Forinstance, where Worm.class is mentioned, change it to Crab.class. Also change thename lookForWorm to lookForCrab. Make sure to update your comments.

Exercise 3.14 Place a crab, three lobsters, and many worms into the world. Run thescenario. Does the crab manage to eat all worms before it is caught by a lobster?

You should now have a version of your scenario where both crabs and lobsters walk aroundrandomly, looking for worms and crabs, respectively.

Now, let us turn this program into a game.

3.6 Keyboard controlTo get game-like behavior, we need to get a player involved. The player (you!) should be able tocontrol the crab with the keyboard, while the lobsters continue to run randomly by themselves,as they already do.

The Greenfoot environment has a method that lets us check whether a key on the keyboard has beenpressed. It is called isKeyDown, and, like the getRandomNumber method that we encountered insection 3.1, it is a method in the Greenfoot class. The method signature is

static boolean isKeyDown(String key)

We can see that the method is static (it is a class method) and the return type is boolean. Thismeans that the method returns either true or false, and can be used as a condition in an if-statement.

3.5 Adding a LobsterWe are now at a stage where we have a crab that walks more or less randomly through our world,and eats worms if it happens to run into them.

To make it a little more interesting, let us add another creature: a lobster.

Lobsters, in our scenario, like to chase crabs.

3.6 Keyboard control | 37

We also see that the method expects a parameter of type String. A String is a piece of text (suchas a word or a sentence), written in double quotes. The following are examples of Strings:

“This is a String”“name”“A”

In this case, the String expected is the name of the key that we want to test. Every key on the key-board has a name. For those keys that produce visible characters, that character is their name, forexample, the A-key is called “A”. Other keys have names too. For instance, the left cursor key iscalled “left”. Thus, if we want to test whether the left cursor key has been pressed, we can write

if (Greenfoot.isKeyDown(“left”)){

...// do something}

Note that we need to write “Greenfoot.” in front of the call to isKeyDown, since this methodis defined in the Greenfoot class.

If, for example, we want our crab to turn left by 4 degrees whenever the left cursor key is beingpressed, we can write


turn(-4);}

The idea now is to remove the code from the crab that does the random turning and also the codethat turns automatically at the world edge and replace them with the code that lets us control thecrab’s turn with our keyboard.

Exercise 3.15 Remove the random turning code from the crab.

Exercise 3.16 Remove the code from the crab that does the turn at the edge of the world.

Exercise 3.17 Add code into the crab’s actmethod that makes the crab turn left wheneverthe left cursor key is pressed. Test.

Exercise 3.18 Add another—similar—bit of code to the crab’s act method that makesthe crab turn right whenever the right cursor key is pressed.

Exercise 3.19 If you have not done so in the first place, make sure that the code thatchecks the key-presses and does the turning is not written directly in the act method, but isinstead in a separate method, maybe called checkKeypress. This method should becalled from the act method.

Tip:

Greenfoot automat-ically saves classesand scenarioswhen their windowsare closed. To keepa copy of interimstages of scenar-ios, use Save ACopy As from theScenario menu.

Try solving the tasks by yourself first. If you get stuck, have a look on the next page. Code 3.5shows the crab’s complete act and checkKeypress methods after this change. The solution isalso available in the book scenarios, as little-crab-3. This version includes all the changes wehave discussed so far.


Concept:

The APIDocumentationlists all classesand methods avail-able in Greenfoot.We often need tolook up methodshere.

You are now ready to have a first try at playing your game! Place a crab, some worms, and a fewlobsters into the world, and see whether you can get all the worms before the lobsters catch you.(Obviously, the more lobsters you place, the harder it gets...)

3.7 Ending the gameOne simple improvement we can make is to end execution of the game when the crab is caughtby a lobster. Greenfoot has a method to do this—we just need to find out what it is called.

To find out what the available methods in Greenfoot are, we can look at the documentation ofthe Greenfoot classes.

In Greenfoot, choose Greenfoot Class Documentation from the Help menu. This will show thedocumentation for all the Greenfoot classes in a Web browser (Figure 3.3).

This documentation is also called the Greenfoot API (for application programmers’ interface).The API shows all available classes and, for each class, all the available methods. You can seethat Greenfoot offers five classes: Actor, Greenfoot, GreenfootImage, MouseInfo, andWorld.

Code 3.5The Crab’s “act”method: Controllingthe crab with thekeyboard

/*** Act - do whatever the crab wants to do.*/

public void act(){

checkKeypress();move();lookForWorm();

}

/*** Check whether a control key on the keyboard has been pressed.* If it has, react accordingly.*/public void checkKeypress(){


turn(-4);}if (Greenfoot.isKeyDown(“right”)){

turn(4);}

}

3.7 Ending the game | 39

Figure 3.3The Greenfoot API ina browser window

The method we are looking for is in the Greenfoot class.

Exercise 3.20 Open the Greenfoot API in your browser. Select the Greenfoot class.In its documentation, find the section titled “Method Summary”. In this section, try tofind a method that stops the execution of the running scenario. What is this methodcalled?

Exercise 3.21 Does this method expect any parameters? What is its return type?

We can see the documentation of the Greenfoot classes by selecting them in the list on theleft. For each class, the main panel in the browser displays a general comment, detailsof its constructors, and a list of its methods. (Constructors will be discussed in a laterchapter.)

If we browse through the list of available methods in the class Greenfoot, we can find amethod named stop. This is the method that we can use to stop execution when the crab getscaught.


We can make use of this method by writing

Greenfoot.stop();

into our source code.

Exercise 3.22 Add code to your own scenario that stops the game when a lobster catchesthe crab. You will need to decide where this code needs to be added. Find the place in yourcode that gets executed when a lobster eats a crab, and add this line of code there.

We will use this class documentation frequently in the future to look up details of methods weneed to use. We will know some methods by heart after a while, but there are always methods weneed to look up.

3.8 Adding soundAnother improvement to our game is the addition of sounds. Again, a method in the Greenfootclass helps us with this.

Exercise 3.23 Open the Greenfoot Class Documentation (from the Help menu), andlook at the documentation of class Greenfoot. Find the details of the method that can beused to play a sound. What is its name? What parameters does it expect?

By looking through the documentation, we can see that the Greenfoot class has a methodcalled playSound. It expects the name of a sound file (as String) as a parameter, and returnsnothing.

Note

You may like to look at the structure of a Greenfoot scenario in your file system. If you look intothe folder containing the book scenarios, you can find a folder for each Greenfoot scenario. Forthe crab example, there are several different versions (little-crab, little-clab-2,little-crab-3, etc.). Inside each scenario folder are several files for each scenario class, andseveral other support files. There are also two media folders: images holds the scenario imagesand sounds stores the sound files.

You can see the available sounds by looking into this folder, and you can make more soundsavailable by storing them here.

In our crab scenario, two sound files are already included. They are called slurp.wavand au.wav.

3.8 Adding sound | 41

Exercise 3.24 Add playing of sounds to your scenario: When a crab eats a worm, playthe “slurp.wav” sound. When a lobster eats the crab, play the “au.wav” sound. To do this,you have to find the place in your code where this should happen.

The little-crab-4 version of this scenario shows the solution to this. It is a version of the projectthat includes all the functionality we have discussed so far: worms, lobsters, keyboard control,and sound (Figure 3.4).

Figure 3.4The crab game withworms and lobsters

We can now easily play one of the sounds by using the following method call:

Greenfoot.playSound(“slurp.wav”);

Try it out!


Exercise 3.25 If you have a microphone on your computer, make your own sounds touse when the worms or the crab get eaten. Record the sounds with any sound recordingprogram, store them in the scenario’s sounds folder, and use them in your code.

3.9 Summary of programming techniquesIn this chapter we have seen more examples of using an if-statement—this time for turning atrandom times and reacting to key presses. We have also seen how to call methods from anotherclass, namely the getRandomNumber, isKeyDown, and playSound methods from theGreenfoot class. We did this by using dot notation, with the class name in front of the dot.

Altogether, we have now seen examples of calling methods from three different places. We cancall methods that are defined in the current class itself (called local methods), method that weredefined in a superclass (inherited methods), and static methods from other classes. The last ofthese uses dot notation. (There is one additional version of a method call: calling methods onother objects—we will encounter that a little later.)

Another important aspect that we explored was how to read the API documentation of an existingclass to find out what methods it has and how to call them.

About sound recording

You can also make your own sounds. Both the sounds included are recorded by simply speakinginto the computer’s microphone. Use one of the many free sound recording programs1, recordyour sound, and save (or export) it as a sound file, in either WAV, AIFF, or AU format. Making yourown sounds is further discussed in Chapter 8.

1 Using an Internet search, you should be able to find several free programs that can record and savesounds. One good program is Audacity (http://audacity.sourceforge.net), but there are many others.

Concept summary

■ When a method we wish to call is not in our own class or inherited, we need to specify the class orobject that has the method before the method name, followed by a dot. This is called dot notation.

■ Methods that belong to classes (as opposed to objects) are marked with the keyword static intheir signature. They are also called class methods.

■ A method definition defines a new action for objects of this class. The action is not immedi-ately executed, but the method can be called with a method call later to execute it.

■ Comments are written into the source code as explanations for human readers. They areignored by the computer.

■ The API Documentation lists all classes and methods available in Greenfoot. We often need tolook up methods here.

http://audacity.sourceforge.net

Documents

Improving the Crab—more sophisticated programming€¦ · method is defined in another class, we need to specify the class or object that has the method, followed by a period (dot),